We have begun to investigate the consequences of PI3K signaling for the physiological effects of thyroid hormone on brain development. Our group has recently identified two tyrosines in the second zinc finger of the mammalian thyroid hormone receptor, TR beta, that are essential for stimulation of PI3K. When either tyrosine is mutated, PI3K stimulation by thyroid hormone is blocked without preventing the receptor from binding to canonical thyroid hormone response elements in DNA and stimulating transcription. To test the relative importance of direct gene regulation and PI3K stimulation in thyroid hormone action during brain development, we made a mutant knock-in strain of mice with a phenylalanine replacing one of these tyrosines, Y147F in TRbeta1 (Y161F in TRbeta2). We made whole-cell recordings under voltage-clamp from CA1 neurons in hippocampal slices from postnatal day 13-17 mice, and measured synaptic responses to stimulation of Schaffer collaterals. In slices from wild-type C57BL/6 mice, acute application of 100 nM 3,3&#8242;,5-triiodo-L-thyronine (T3) rapidly produced a sustained 50% increase in excitatory postsynaptic currents (EPSC) at -70 mV, which were blocked completely by 10 uM CNQX. When glutamatergic receptors were blocked pharmacologically, T3 also produced a sustained depression of inhibitory postsynaptic currents (IPSC) at 0 mV, which were blocked by 1 uM GABAzine. However, in slices from mutant animals, both EPSC and IPSC amplitudes were smaller, and the acute effects of thyroid hormone on synaptic responses were eliminated completely. In addition high frequency stimulation at 100Hz for 1sec also failed to induce LTP in the slices from mutant animals. In summary, thyroid hormone signaling through PI3K appears to be essential for postnatal plasticity of both excitatory and inhibitory synapses on mouse hippocampal pyramidal neurons. Thus, disruption of thyroid hormone signaling through PI3K by environmental toxicants could be an important mechanism for environmental effects on human cognitive development. In collaboration with Dr. Sheryl Moy in the Dept. Psychiatry at UNC Chapel Hill, we are testing the effects of this mutation on the mice's behavior in classical learning and memory paradigms. In collaboration with Sandra Rossie in the Dept. Biochemistry at Purdue University, we are studying the role of PP5 as a downstream effector of the Rac GTPase, to understand which PI3K-dependent effects of thyroid hormone on neuronal plasticity and survival require PP5.